1. Field of the Invention
This invention pertains to mixing apparatus, and more particularly to apparatus for continuously blending powders with a liquid.
2. Description of the Prior Art
Exemplary apparatus for blending liquids and particulate solids is disclosed in U.S. Pat. No. 3,606,270. That blender has met with considerable commercial success.
However the blender of the U.S. Pat. No. 3,606,270 is somewhat limited in its ability to handle high viscosity liquid-powder end products. That is because, as the end product viscosity increases, the blender outlet pressure decreases and the vacuum created in the eye of the blender impeller becomes quite low. Consequently, the amount of powder drawn into the impeller decreases and yields a higher ratio of liquid to powder than is desired. To alleviate that problem, the screen that normally surrounds the impeller periphery may be removed, but then the mixing action produced by the liquid and powder flowing through the screen becomes unavailable. Accordingly, blender performance and product consistency occasionally become unacceptable at high end product viscosities.
Thus, a need exists for a blender capable of handling high viscosity liquid-powder end products and wherein there is not a significant drop in vacuum at the inlet as more powder is added so that the addition rate of powder remains constant for a wide range of applications from low to high viscosity mixing and blending and with high flow rates. With prior art single stage blenders the vacuum typically decreases below 20 to 25 inches of mercury and the powder addition rate goes down with increases in viscosity. This results in a decrease in the rate of addition of powder and a longer period of time to mix the powder. The longer mixing time and resulting churning action causes aeration of the mix and undesirable foam.